Indoleamine 2,3-dioxygenase 1 (IDO1, EC 1.13.11.42, also known as indoleamine 2,3-dioxygenase) is the first and rate-limiting enzyme in the tryptophan-kynurenine pathway that degrades the essential amino acid L-tryptophan (L-Trp) to N-formal-kynurenine, which can be subsequently metabolized through a series of steps to form NAD. IDO1 enzyme is expressed in the placenta, the mucosal and lymphoid tissues, and in inflammatory lesions (Yamazaki F, et. al., Biochem J. 1985; 230:635-8; Blaschitz A, et. al., PLoS ONE. 2011; 6:e21774). In the latter two, it is expressed primarily by antigen-presenting cells (APC), mainly dendritic cells (DC) and macrophages, and in cells exposed to interferon-gamma (IFNγ) and other pro-inflammatory stimuli. In human cells, the depletion of L-Trp resulting from IDO1 activity as well as the production of a series of immunoregulatory metabolites, collectively known as “kynurenines”, can suppress the proliferation and differentiation of effector T cells [Frumento G, et. al., (2002), Journal of Experimental Medicine 196: 459-468], and markedly enhance the suppressor activity of regulatory T cells [Sharma M D, et al. (2009), Blood 113: 6102-6111]. As a result, IDO1 controls and fine-tunes both innate and adaptive immune responses [Grohmann U, et al. (2002), Nature Immunology 3: 1097-1101] under a variety of conditions, including pregnancy [Munn D H, et al. (1998), Science 281: 1191-1193], transplantation [Palafox D, et al. (2010), Transplantation Reviews 24: 160-165], infection [Boasso A, et al. (2009), Amino Acids 37: 89-89], chronic inflammation [Romani L, et al. (2008), Nature 451: 211-U212], autoimmunity [Platten M, et al. (2005), Science 310: 850-855], neoplasia, and depression [Maes M, et. al., Life Sci. 2002 6; 71(16): 1837-48; Myint A M, et. al., (2012), Journal of Neural Transmission 119: 245-251].
Several lines of evidence suggest that IDO is involved in induction of immune tolerance. The immunosuppressive effect of IDO1 was demonstrated first in a mouse model of fetal protection against maternal immune rejection. Treatment of pregnant mice with a tryptophan analog that inhibits IDO1, which is constitutively expressed in the placenta, resulted in T cell-mediated rejection of allogeneic embryos [Munn D H, et al. (1998), Science 281: 1191-1193]. Subsequent studies developed this concept as a mechanism to defeat immune surveillance in cancer (reviewed in [Prendergast G C (2008), Oncogene 27(28):3889-3900; Munn D H, et. al., (2007), J Clin Invest 117(5):1147-1154]). Indoleamine 2,3-dioxygenase is widely overexpressed in tumor cells where it is has been associated predominantly with poor prognosis [Uyttenhove C, et. al., (2003), Nat Med 9(10):1269-1274; Liu X, et. al., (2009), Curr Cancer Drug Targets 9(8):938-95]. Expression of IDO by immunogenic mouse tumor cells prevents their rejection by preimmunized mice [Uyttenhove C. et. al., Nat Med. 2003 October; 9(10):1269-74. Epub 2003 Sep. 21]. IDO activity is shown to suppress T cells [Fallarino F, et. al., (2002), Cell Death Differ 9:1069-1077; Frumento G, et. al., (2002), J Exp Med 196(4):459-468; Terness P, et. al., (2002), J Exp Med 196(4):447-457] and NK cells [Della Chiesa M, et. al., (2006), Blood 108(13):4118-4125], and also that IDO was critical to support the formation and activity of Tregs [Fallarino F, et. al., (2003), Nat Immunol 4(12):1206-1212] and myeloid-derived suppressor cells (MDSCs) [Smith C, et. al., (2012), Cancer Discovery 2(8):722-735]. It has been suggested that the efficacy of therapeutic vaccination of cancer patients might be improved by concomitant administration of an IDO inhibitor [Uyttenhove C. et. al., Nat Med. 2003 October; 9(10):1269-74. Epub 2003 Sep. 21]. It has been shown that the IDO inhibitor, 1-MT, can synergize with chemotherapeutic agents to reduce tumor growth in mice, suggesting that IDO inhibition may also enhance the anti-tumor activity of conventional cytotoxic therapies [Muller A J, et. al., Nat Med. 2005 March; 11(3):312-9]. It has been shown that IDO inhibitors can synergize with anti-CTLA-4 antibody or anti-PDL-1 antibody in inhibiting tumor growth in mouse models [Holmgaard R B, et. al., J Exp Med. 2013 Jul. 1; 210(7):1389-402; Spranger S, et. al., J Immunother Cancer. 2014, 2:3].
It has been proposed that IDO is induced chronically by HIV infection, and is further increased by opportunistic infections, and that the chronic loss of Trp initiates mechanisms responsible for cachexia, dementia and diarrhea and possibly immunosuppression of AIDS patients [Brown, et al., 1991, Adv. Exp. Med. Biol., 294: 425-35]. To this end, it has recently been shown that IDO inhibition can enhance the levels of virus-specific T cells and, concomitantly, reduce the number of virally infected macrophages in a mouse model of HIV [Portula et al., 2005, Blood, 106:2382-90]. Simian Immunodeficiency Virus (SIV) is very similar to Human Immunodeficiency Virus (HIV) and it is used to study the condition in animal models. In both HIV and SIV, the level of virus in the blood, or ‘viral load’, is important because when the viral load is high, the disease progresses and it depletes the patient's immune system. This eventually leads to the onset of Acquired Immune Deficiency Syndrome (AIDS), where the patient cannot fight infections which would be innocuous in healthy individuals. It has also been reported that monkeys with the simian form of HIV treated with an IDO inhibitor, called D-1mT alongside Anti-Retroviral Therapy (ART), reduced their virus levels in the blood to undetectable levels, therefore when combined with ARTs, IDO inhibitors may help HIV patients not responding to treatment in the future [Adriano Boasso, et. al., J. Immunol., April 2009; 182: 4313-4320].
In light of the experimental data indicating a role for IDO in immunosuppression, tumor resistance and/or rejection, chronic infections, HIV-infection, AIDS (including its manifestations such as cachexia, dementia and diarrhea), autoimmune diseases or disorders (such as rheumatoid arthritis) and depression, therapeutic agents aimed at suppression of tryptophan degradation by inhibiting IDO activity are of interests. Inhibitors of IDO can be used as effective cancer therapy as they could reverse the immunosuppressive effects of tumor microenvironment and activate anti-tumor activity of T cells. IDO inhibitors could also be useful in activation of immune responses in HIV infection. Inhibition of IDO may also be an important treatment strategy for patients with neurological or neuropsychiatric diseases or disorders such as depression. The compounds, compositions and methods herein help meet the current need for IDO modulators.
Tryptophan 2,3-dioxygenase (TDO, EC 1.13.11.11) catalyzes the same Trp degradation reaction as IDO1. TDO is primarily expressed in the liver in humans, where acts as the main regulator of systemic tryptophan levels. More recently, TDO was also found to be expressed in the brain, where it may regulate the production of neuroactive tryptophan metabolites such as kynurenic acid and quinolinic acid [Kanai M, et. al., Mol Brain 2009; 2:8]. Two recent studies [Opitz C A, et. al., Nature 2011; 478:197-203; Pilotte L, et. al., Proc Natl Acad Sci USA. 2012, 109(7):2497-502] point to the significance of TDO activity in certain cancers where it is expressed constitutively (particularly malignant glioma, hepatocellular carcinoma, melanoma, and bladder cancer). Functional studies in human tumors indicate that constitutive TDO enzymatic activity is sufficient to sustain biologically relevant tryptophan catabolism that is capable of suppressing antitumor immune responses [Opitz C A, et. al., Nature 2011; 478:197-203; Pilotte L, et. al., Proc Natl Acad Sci USA. 2012, 109(7):2497-502]. TDO expression by tumors is reported to prevent rejection by immunized mice. A specific TDO inhibitor is shown to restore the ability of mice to reject TDO-expressing tumors without causing significant toxicity [Pilotte L, et. al., Proc Natl Acad Sci USA. 2012, 109(7):2497-502]. Therefore, inhibitors of TDO can potentially be used as a single agent or in combination with other anti-cancer therapies to treat a variety of human cancers.
Small molecule inhibitors of IDO are being developed to treat or prevent IDO-related diseases such as those described above. Fox example, PCT Publication WO 99/29310 reports methods for altering T cell-mediated immunity comprising altering local extracellular concentrations of tryptophan and tryptophan metabolites, using an inhibitor of IDO such as 1-methyl-DL-tryptophan, p-(3-benzofuranyl)-DL-alanine, p-[3-benzo(b)thienyl]-DL-alanine, and 6-nitro-L-tryptophan) (Munn, 1999). Reported in WO 03/087347, also published as European Patent 1501918, are methods of making antigen-presenting cells for enhancing or reducing T cell tolerance (Munn, 2003). Compounds have indoleamine-2,3-dioxygenase (IDO) inhibitory activity are further reported in WO 2004/094409; WO 2006/122150; WO 2009/073620; WO 2009/132238; WO 2011/056652, WO 2012/142237; WO 2013/107164; WO 2014/066834; WO 2014/081689; WO 2014/141110; WO 2014/150646; WO 2014/150677; WO 2015006520; WO 2015/067782; WO 2015/070007; WO 2015/082499; WO 2015/119944; WO 2015/121812; WO 2015/140717; WO 2015/173764; WO2015/188085; WO 2016/026772; US 2015328228 and US 2015266857. In particular, the compounds of WO 2012/142237 and WO 2014/159248 encompass a series of tricyclic imidazoisoindoles with potent IDO inhibitory activity.
Some substituted imidazo[1,5-a]pyridines are known in the literatures. For example, WO 2008110523 A1 (published on Sep. 18, 2008) has disclosed imidazo[1,5-a]pyridines as glutaminyl cyclase inhibitors; GB2174094A (published on Oct. 29, 1986) discloses imidazo [1,5-a] pyridine derivatives as thromboxane synthetase inhibitors; and JP1997071586A (published on Mar. 18, 1997) discloses imidazo[1,5-a]pyridines as inhibitors of the aldosterone biosynthetic enzyme cytochrome P450C18 for the treatment of primary or secondary aldosteronism, renal hypertension and so on.
However, no imidazo[1,5-a]pyridine has been reported as an IDO/TDO inhibitor. Disclosed herein are novel 5 or 8-substituted imidazo[1,5-a]pyridines exhibiting IDO, in particular IDO1, TDO, or IDO/TDO dual inhibitory activity. The inventors of the present application have unexpectedly found that substitution of hydroxyl group on the chiral α-carbon atom attached to position 5 or 8 of the imidazo[1,5-a]pyridine structure and/or ortho or meta substitution in relation to the hydroxyl-substituted chiral α-carbon atom on the pyridine moiety of the imidazo[1,5-a]pyridine structure impart unexpected enzymatic and cellular activity to the novel 5 or 8-substituted imidazo[1,5-a]pyridines disclosed herein.